First responders, soldiers, and other wearers often work in hazardous environments that require personal protective equipment (PPE). These hazardous environments may contain airborne contaminants that require respiratory protection. Some existing respirators include an End of Service Life Indicator (ESLI) to notify the users of the end of the useful life of the cartridge, where the cartridge life is the duration during which harmful gases do not exceed the permissible exposure limits at the user.
On Apr. 8, 1998, the Occupational Safety and Health Administration (OSHA) issued revised regulations for ESLI indicators. Specifically 29 C.F.R. 1910.134 requires the following: “For protection against gases and vapors, the employer shall provide: . . . An air-purifying respirator, provided that: The respirator is equipped with an end-of-service-life indicator (ESLI) certified by NIOSH for the contaminant; or If there is no ESLI appropriate for conditions in the employer's workplace, the employer implements a change schedule for canisters and cartridges that is based on objective information or data that will ensure that canisters and cartridges are changed before the end of their service life.” If an ESLI is appropriate for the workplace conditions, the respirator must be equipped with an ESLI.
To indicate when a cartridge is nearing the end of its useful life, the ESLI must include a notification mechanism. However, an audible notification for ESLI, equipment failure, or a situational alarm (e.g. low air) may interfere with the user's ability to hear ambient noises. In the case of firefighting, the firefighter needs to be able to hear what is going on in the surrounding environment, and cannot afford to have an in-ear earpiece that interferes with the ambient sound. However, without an in-ear speaker, the alarms will be inaudible in loud environments. There is a tradeoff: in-ear speakers do not allow ambient awareness, out-of-ear speakers may not allow alarm awareness.